docs: def find_invalid_filenames(filenames, repository_root):
      """Find files that does not exist, are not in the repo or are directories.
  
      Args:
        filenames: list of filenames to check
        repository_root: the absolute path of the repository's root.
  
      Returns: A list of errors.
      """
      errors = []
      for filename in filenames:
          if not os.path.abspath(filename).startswith(repository_root):
              errors.append((filename, 'Error: File %s does not belong to '
                             'repository %s' % (filename, repository_root)))
          if not os.path.exists(filename):
              errors.append((filename,
                             'Error: File %s does not exist' % (filename, )))
          if os.path.isdir(filename):
              errors.append((filename,
                             'Error: %s is a directory. Directories are'
                             ' not yet supported' % (filename, )))
  
      return errors